1. Technical Field
The present invention generally relates to integrated circuits, and more specifically to those integrated circuits that use copper for the interconnect layers. Still more specifically, this invention relates to integrated circuits that incorporate fuse technology.
2. Background Art
As microprocessor makers move towards new generations of processes, e.g., the 0.18 .mu.m CMOS process, new materials for the interconnect layers must be chosen to decrease their resistance. The use of copper in place of the traditional aluminum is considered by most chip manufacturers.
Copper, however, has one major disadvantage for integrated circuits: it has a higher mobility in silicon than aluminum. If opper atoms are left on their own, they would spread throughout the chip, ultimately contaminating the transistors and preventing them from operating properly.
An important feature of integrated circuit designs is fuse technology. That is, a large number of integrated circuit designs now include fuses to implement certain features or to achieve desired results. For example, for many years dynamic random access memories (DRAMs) have included spare word lines and bit lines that can be substituted through fuse technology for faulty lines. On some integrated circuits, fuses are used to customize the chip function after manufacture. Fuses may also be used to trim precision components such as resistors or capacitors.
A fuse has two possible states: the first state, when the fuse is intact; and the second or "blown" state, when the fuse is blown or broken down. A fuse is typically blown through laser ablation or by forcing an excessive current through a narrow on-chip fuse wire. When either of these techniques is used, not only is the dielectric encapsulation or protective covering around the fuse ruptured, which allows the metallic or similar material of the fuse to escape, but any other anti-corrosive material around the fuse is also ruptured.
Fuses are typically formed in the topmost layer metal wires where they can be easily accessed for laser programming and from where the metallic material of the fuse can easily escape. In the past, the material used for for fuses has been aluminum, which produces an oxide that is largely self passivating, thus preventing the fuse from "regrowing". Examples of this type of and similar types of fuses are found in the following U.S. patents: U.S. Pat. No. 5,291,139, "Circuit for Detection of the State of an Integrated Circuit Fuse in a Balanced Fuse Configuration", issued March 1994 to Fruhauf et al., and U.S. Pat. No. 5,404,049, "Fuse Blow Circuit", issued April 1995 to Canada et al.
As already mentioned before, future integrated circuits are progressing towards the use of other types of metals for wiring and fuses, such as copper, to allow for a smaller resistance in the circuit. Unfortunately, though, copper does not produce self-passivating oxides and thus is subject to dendritic growth, which can "regrow" a blown fuse, defeating the purpose of the fuse. In Linley Gwennap, "IC Makers Confront RC Limitations", Microprocessor Report, Aug. 4, 1997, there is disclosed a barrier solution that envelopes all copper traces inside a protective sheath. This cladding keeps the copper away from all silicon structures and thus prevents oxidation. This solution is a major progress when intending to use copper instead of aluminum. However, since there is no barrier between the two ends of the blown fuse any longer, and the distance between those ends is very small, the fuse could repair itself because of the mobility of the copper atoms.
Exposed copper is also subject to corrosion. Corrosion may enter the copper at the ruptured site and proceed to follow the wire to a junction where it will interrupt a device or circuit that is intended to remain conductive. Although the aforementioned patents disclose methods to detect the integrity of fuses, they do not disclose a method for preventing dendritic growth or corrosion in a blown fuse.
U.S. patent application 914,857, filed Aug. 19, 1997 and assigned to the same assignee as the present invention, discloses a voltage limiting circuit for fuse technology. An integrated circuit comprises a fuse having two terminals and a voltage limiting circuit that is coupled to the terminals. The voltage limiting circuit is responsive to a fuse blow through a low impedance sensing circuit, and then minimizes the voltage across the fuse gap that is created by the fuse blow. Thus, dendritic growth as well as corrosion in copper or similar types of fuses is reduced.
However, this solution still suffers from several drawbacks. First, the voltage is only limited to a few hundreds of mV and the current is in the nA to .mu.A range. Since there is still a voltage present, the possibility remains that copper atoms will spread over the very small distance between the two ends of the blown fuse and thus "regrow" the fuse. This will be the more damaging, the more advanced the processes will become.
Secondly, the impedance of the proposed sensing circuit is no larger than is needed to reliably sense the state of the fuse. This means that the impedances and the sensing circuit must be well designed to function correctly and will have to change from technology to technology.
Finally, the solution favors two current sources. The design of these current sources will consume a lot of silicon.